MECHANICAL SEPARATION 293 FIG. 17.—Portland valve plate. The stationary port B has its inner face carefully ground so as to make intimate contact with the revolving plate A. The operation of the air passages is very simple. Port C connects all the panels which are submerged in the pulp and is under vacuum. D is a port into which all the pipes from the ascending panels open. E connects with all panels in the earlier part of their descent. D and E are under suction. F connects with each panel in succession just before it reaches the scraper C, Fig. 15, admitting air under pressure to dislodge the cake. G is another pressure port connecting with each panel after it passes the scraper to give the filter medium a cleansing action before becoming submerged. The valve plate of the Oliver filter is on similar lines. If the cake is the valuable material to be separated ports C, D and E are all connected so that the worthless filtrate will go towards a common discharge point. If the filtrate is valuable D and E will carry wash water of two different strengths which may have different destination from one another and the strong liquor pulled through the port C. In the cyanide process for gold extraction weak cyanide wash may issue from D carrying with it further gold values from the cake while through E may pass the final wash water to remove any remaining cyanide. Figure 18 illustrates the arrangement of the filtering medium on the Portland filter and shows the wooden frame construction. Where acid or caustic liquors have to be filtered the machines must be made of special materials and the filtering medium must be chosen from materials which will best withstand chemical action. The back of the panel is constructed of redwood staves held together with steel tongues. Each panel is made a complete unit by angle-iron segments bent to the curve of the drum and secured to the staves. The working face of each panel is provided with a molding Bj making a recess in which the wire cloth, C, lies separated from the face of the drum. Within the space enclosed by the moulding, wooden strips, D, are fastened, these acting as supports for the wire cloth and providing drainage space for the liquid. The pipe which serves each panel has two connections with this space through the back, so that drainage is complete whether the panel is rising from the tank or descending into it. Grooves E lead to the pipe connections and assure unobstructed flow. Upon the wire cloth is placed burlap, indicated at F, and the surface fabric, G, surrounds this in turn. The seal between the panels is formed in the space //. The canvas though continuous does not pass uninterruptedly from one panel to another but is carried down in the groove, //, and held there by a small strip which is squeezed into the canvas. This arrangement seals the panels from one another. To hold the filter medium in place and protect it from wear by the scraper it is wound with wire. Figure 16 shows at Z a threaded shaft which assists in wire winding. The threaded shaft is belted up to the drum shaft. Wiring starts from the side of the drum and after the wire from the reel has passed through a tension clip or brake and around a groove of the threaded shaft it is secured to the side of the drum. As the latter revolves the threaded slot keeps it properly spaced and the brake insures FIG. 18.—Detail of Portland filter.